with Android P's Clang versoin: 6.0.2, and Android NDK's Clang version 8.0.2

Clang will report below error:
criu/compel/include/uapi/compel/sigframe-common.h:55:34: error: expected member name or ';' after declaration specifiers
        int                     __unused[32 - (sizeof (k_rtsigset_t) / sizeof (int))];
        ~~~                             ^

it takes __unused as an attribute, not a varible, chang to _unused, pass compile.

Cc: Chen Hu <hu1.chen@intel.com>
Signed-off-by: Zhang Ning <ning.a.zhang@intel.com>
Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>